The invention relates generally to ignition systems, and more particularly to apparatus and methods for detecting and indicating the occurrence and type of discharges from an exciter circuit.
Conventional ignition systems are well known and typically include an exciter circuit having an energy storage device such as a capacitor and a circuit for charging the capacitor, one or more igniter plugs circuit, and a switching mechanism as part of a discharge circuit connected between the capacitor and the igniter. In aerospace applications, the switching mechanism commonly is a spark gap, or more recently solid state switches such as SCRs. A control circuit can be provided to control when the switching mechanism is triggered so that the energy stored in the capacitor can be discharged across the igniter plug. During the time that the switching device is open, the capacitor is charged by the charging circuit. The control circuit may include a timer circuit to control the spark rate.
It is often desirable to know whether the ignition system is operating properly, particularly to know if the spark rate is being maintained. For example, spark rates can be significantly affected by operating temperature excursions or variations of input voltage or frequency. Also, various failure modes within the discharge circuits can prevent proper discharge of current through the igniter. Accordingly, many ignition diagnostic systems use a current transformer to detect discharge, typically through the high tension lead or return lead. The current transformer includes a wire coil on a high permeability core that surrounds the current lead. Discharge current through the ignition system cables induces a current in the transformer that can then be detected by the diagnostic system because the induced current is related to the occurrence of a discharge current. The current transformer, therefore, provides a way to detect the occurrence of a discharge.
However, such discharge detection schemes essentially operate as a go/no-go type diagnostic signal. The signal can indicate whether a spark discharge occurred or not, but cannot provide any further information as to what may have caused the igniter not to fire.
In many aerospace applications, more than one exciter circuit may be used per engine for ignition. In such circumstances, a simple go/no-go type diagnostic signal does not provide sufficient information when a spark discharge fails to occur.
Although multiple diagnostic signals could be used, this approach is unacceptable in modern engines because of the added wiring and weight. Multiple diagnostic signals also increase the complexity of the electronics needed to interpret the diagnostic signals.
The objectives exist, therefore, for apparatus and methods for producing diagnostic signals that can indicate whether exciter circuit discharges occur and the nature of the discharges. Such apparatus and methods preferably should produce such diagnostic signals using a single diagnostic output to simplify monitoring the signals.